Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
In recent years there has been a rapid increase in user demand for higher data rate information and communication services. A major driver for this demand is the increase in popularity of high data rate media streaming services such as video-on-demand and voice over IP (VoIP), which require fast broadband internet services. Capability for these high bandwidth services is increasingly being provided by optical networks incorporating dense wavelength division multiplexing (DWDM) schemes. Such schemes involve combining multiple wavelength channels into a single optical signal and transmitting the signal over high bandwidth optical fibers.
A limiting factor in most optical networks is the ability to rapidly and dynamically route each optical channel from its constituent multiplexed signal within an optical fiber to other fibers and eventually to a desired end user. Traditionally such routing or switching was performed in the electrical domain. However, electrical switching is relatively slow due to the necessary conversion from the optical domain to the electrical domain and back again. With increased bandwidth demands, switching has been shifted to the optical domain where much higher speeds can be realized.
Current optical switching is typically performed by wavelength selective switches (WSSs), which currently allow reconfigurable switching of wavelength channels from a single input fiber to one of a number of output fibers to drop or reroute particular wavelength channels. WSSs generally include a demultiplexing module complemented with a corresponding multiplexing module, which collectively perform adding and dropping of individual optical channels from DWDM signals. The reconfigurable nature of these switches makes them favorable for dynamically routing particular wavelength channels across a network depending on user demand, network disruptions and other factors.
As demand for higher network capacity increases, a need for the ability to simultaneously and reconfigurably switch wavelength channels between multiple input ports and multiple outputs is becoming apparent. When compared to existing single input-multiple output devices, this design is advantageous in reducing the number of required switching devices in the network and improving network reliability as signals can be easily routed away from individual problematic fibers.
More recent implementations seek to route wavelength channels in a “colorless”, “directionless” and “contentionless” manner. A colorless WSS is a device that can route a channel independent of its wavelength. That is, the device has no hardware constraints on wavelength routing. Directionless WSS devices are capable of routing a particular wavelength channel from any input port to any output port in any direction. A contentionless WSS design permits routing of multiple wavelength channels having the same wavelength without suffering significant crosstalk.
A further desirable feature of a WSS is spectral flexibility. The hardware designs of most WSS devices restrict channel routing to wavelength grids having a fixed channel plan. That is, channels having a fixed spectral spacing and spectral width. WSS designs having spectral flexibility permit routing that is not limited to a particular channel plan. Therefore, these designs are capable of readily adapting to changes in channel plans that can occur over time.
U.S. Pat. No. 7,397,980 to Frisken, entitled. “Dual-Source Optical Wavelength Processor” discloses an optical switch capable of reconfigurably switching channels from two separate input sources to a number of output ports. This is achieved by first transmitting the two vertically displaced signals coincidentally onto a grism to spatially separate the multiplexed wavelength channels in a horizontal dispersion dimension. The dispersed wavelength channels of each signal are then incident onto separate wavelength processing regions of a liquid crystal on silicon (LCOS) device. The wavelength processing regions are defined by two vertically separated subsets of the LCOS pixels. Each incident wavelength signal is focused in the dispersion dimension but collimated in the vertical plane such that the signals are incident onto a vertically disposed array of LCOS pixels. By applying predetermined independent phase manipulation functions to specific vertical rows of LCOS pixels (corresponding to the position of each wavelength channel), the phase front of each channel can be directionally controlled in the vertical plane. This allows independent steering of each wavelength channel to select a desired output port for each respective channel.
In Frisken, two separate inputs are essentially independent from each other and wavelength channels from each input are treated separately. That is, outputs are hardwired to a given input. Aside from desiring flexibility in switching between inputs and outputs, extending the number of inputs to higher numbers in itself is not straightforward. In particular, as the number of available pixels on an LCOS device is limited, adding more wavelength processing regions comes at the cost of sacrificing the number of pixels available for each processing region. The smaller available number of pixels provides difficulties in steering to peripheral ports, particularly when a larger number of output ports are included. This peripheral steering is required to dynamically allocate each output port to a given input port without sacrificing the advantage of flexible channel allocation through a fixed array.
US Patent Application Publication 2010/0172646 A1 to Colbourne, entitled “M×N Wavelength Selective Optical Switch” discloses an optical device for switching a number (K) of individual wavelength channels from one of an arbitrary number (M) of input fibers to one of an arbitrary number (N) of output fibers. A diffraction grating is used to spatially separate the individual wavelength channels of each optical signal. The wavelength channels are incident onto individual mirrors of a first MEMS array having M×K individual mirrors before being coupled back through the diffraction grating where the channels are spatially recombined but angularly separated. The recombined channels are transmitted through a switching lens which individually directs each wavelength signal, depending on angle set by the first MEMS array, to a particular mirror of a second MEMS array having N mirrors. Each MEMS mirror of the second MEMS array is associated with a particular output fiber and the particular wavelength channel coupled to that fiber is determined by the angle of each MEMS mirror.
The implementation disclosed in Colbourne is not a spectrally flexible architecture. Specifically, due to the fixed positions of the MEMS mirrors, this arrangement is not capable of handling flexible spectral grids. That is, the MEMS mirrors are each disposed in a predetermined fixed location to route a particular wavelength channel based on a fixed channel spacing. If the channel spacing or channel bandwidth is varied, the routing of the wavelength channels becomes much less efficient.
In Colbourne, individual wavelength channels can be routed from any input fiber to any output fiber in a colorless and directionless manner by controlling the tilt angles of two MEMS arrays. However, as seen in FIG. 2A of that document, each separated wavelength channel is reimaged at an intermediate focal plane before being coupled to an output fiber. At the intermediate focal plane, each channel of a common input fiber shares a common spatial spot but has a different trajectory angle. Switching to a desired output fiber is performed by a switching lens of small focal length to convert the angle θ to a displacement. Therefore, the maximum separation of the wavelength channels is approximately limited to the focal length of this switching lens multiplied by the angle. Consequently, using such a configuration in cases where the switching matrix is an LCOS device, switching to distant output ports is limited, thereby practically limiting this device to small numbers of output fibers.
Therefore, there is a desire to provide a wavelength selective switch that can efficiently and reconfigurably route wavelength channels from a plurality of input ports to a plurality of output ports in a spectrally flexible manner.